Optical signal monitor, optical wavelength multiplexing transmitter, and method for monitoring optical signal

ABSTRACT

An optical signal monitor, including: a storage that holds a threshold value set for each of determination areas having a bandwidth set in accordance with an average grid of dummy light; a measurement section that sequentially measures an optical intensity of an inputted wavelength-multiplexed optical signal with respect to each of measurement areas obtained by dividing the determination area into areas with a bandwidth sufficiently smaller than a grid width of a monitoring-target optical signal composing the wavelength-multiplexed optical signal, and output measured values; and a section that determines that dummy light corresponding to the determination area needs introducing if each of measured values in the determination area is smaller than a threshold value, and, determines that dummy light corresponding to the determination area does not need introducing if at least one of the measured values in the determination area is equal to or larger than the threshold value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.17/032,992, filed Sep. 25, 2020, which is a Continuation of U.S.application Ser. No. 16/440,002, filed Jun. 13, 2019, and issued as U.S.Pat. No. 10,805,003, which is a Continuation of U.S. application Ser.No. 16/039,739, filed on Jul. 19, 2018, and issued as U.S. Pat. No.10,404,366, which is a Continuation of Ser. No. 15/551,810, filed onAug. 17, 2017, and issued as U.S. Pat. No. 10,056,976, which is aNational stage of International Application No. PCT/JP2016/001102 filedon Mar. 1, 2016, which claims priority benefit from Japanese PatentApplication 2015-042535 filed on Mar. 4, 2015, the contents of all ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to optical signal monitors, opticalwavelength multiplexing transmitters, and an methods for monitoringoptical signal and, in particular, to an optical signal monitor, anoptical wavelength multiplexing transmitter, and a method for monitoringoptical signal that process a wavelength-multiplexed optical signalcomposed of optical signals in which a plurality of frequency grids aremixed.

BACKGROUND ART

Recently, services to handle large-capacity contents such as a videohave been rapidly expanded because of the growth of the Internet and thelike. This leads to a growing demand for a large-capacity backbonenetwork; consequently, it becomes important to use a finite opticalspectral region more efficiently. One of technologies to use an opticalspectral region efficiently is a wavelength division multiplexing (WDM)transmission system. In the WDM system, a plurality of signal lightbeams with their center wavelengths different from each other arecombined, amplified to a desired level in an optical amplifier, and thenoutput to an optical fiber transmission line.

In general, an optical amplifier has a wavelength-dependent gain, andparticularly, the wavelength dependence prominently arises if an inputlevel of an optical signal is reduced. In this case, an output level ofan optical signal becomes dependent on wavelength. Patent Literature 1discloses a technology to maintain a constant gain of an opticalamplifier by disposing a correction light source and, if the number ofoptical signals composing a WDM signal decreases, by introducingcorrection light into the WDM signal instead. FIG. 11A illustrates anexample of an output level without correction light introduced, and FIG.11B illustrates an example of an output level with correction lightintroduced, if the number of optical signals decreases.

In FIG. 11A and FIG. 11B, if optical signals of wavelengths λ3 to λ7 arelost, the wavelength dependence of the gain of an optical amplifierprominently arises if the correction light is not introduced, and outputlevels of main signal light with λ1, λ2, and λ8 becomes dependent onwavelength. In contrast, the gain of the optical amplifier can beflattened by introducing correction light with λa to λc corresponding tothe lost optical signals with the wavelengths λ3 to α7; consequently,the output levels of main signal light with λ1, λ2, and λ8 becomeunaffected by the gain.

CITATION LIST Patent Literature

[PTL 1] Japanese Patent Application Laid-Open Publication No. 2007-12767

SUMMARY OF INVENTION Technical Problem

In order to use an optical spectral region more effectively, it isproposed to optimize frequency grids of optical signals in accordancewith transmission technologies such as a transmission rate and amodulation method. The technology disclosed in Patent Literature 1cannot deal with a situation where a plurality of optical signals arelost each of which has a different frequency grid because the technologydoes not make the assumption that a WDM signal is composed of opticalsignals in which a plurality of frequency grids are mixed.

The present invention has been made in view of the above-describedproblem, and it is an object of the present invention to provide anoptical signal monitor, an optical wavelength multiplexing transmitter,and an method for monitoring optical signal that can make a judgment ofnecessity to introduce dummy light having a highly accurate grasp of achange in an input level influencing a gain even though awavelength-multiplexed optical signal is composed of optical signals inwhich a plurality of frequency grids are mixed.

Solution to Problem

In order to achieve the above-described object, an optical signalmonitor according to an exemplary aspect of the present inventionincludes a storage means for holding a threshold value i (i=1 to N) setfor each of N determination areas i (i=1 to N) having a bandwidth n setin accordance with an average grid of dummy light; a measurement meansfor sequentially measuring an optical intensity of an inputtedwavelength-multiplexed optical signal with respect to each ofmeasurement areas 1 to M obtained by dividing the determination area iinto M areas with a bandwidth m (M=n/m) sufficiently smaller than a gridwidth of a monitoring-target optical signal composing thewavelength-multiplexed optical signal, and outputting M×N measuredvalues; and a determination means for determining that dummy lightcorresponding to the determination area i needs introducing if each of Mmeasured values in the determination area i is smaller than a thresholdvalue i, and, determining that dummy light corresponding to thedetermination area i does not need introducing if at least one of the Mmeasured values in the determination area i is equal to or larger thanthe threshold value i.

In order to achieve the above-described object, an optical wavelengthmultiplexing transmitter according to an exemplary aspect of the presentinvention includes a plurality of optical output means for outputtingoptical signals with wavelengths differing from each other having aplurality of grids; a wavelength multiplex means forwavelength-multiplexing the optical signals output from the plurality ofoptical output means, and outputting a wavelength-multiplexed opticalsignal; an optical splitting means for splitting thewavelength-multiplexed optical signal having output into two signals,and outputting split wavelength-multiplexed optical signals; theabove-described optical signal monitor configured to receive input ofone of the split wavelength-multiplexed optical signals and make ajudgment of necessity to introduce dummy light into a correspondingdetermination area i based on an optical intensity of the one of thesplit wavelength-multiplexed optical signals; a dummy light sourceconfigured to output dummy light corresponding to the determination areai based on the judgment; and an optical coupling means for couplinganother of the split wavelength-multiplexed optical signals with thedummy light having output and outputting a transmission signal.

In order to achieve the above-described object, an method for monitoringoptical signal according to an exemplary aspect of the presentinvention, using a threshold value i (i=1 to N) set for each of Ndetermination areas i (i=1 to N) having a bandwidth n set in accordancewith an average grid of dummy light, includes measuring sequentially anoptical intensity of an inputted wavelength-multiplexed optical signalwith respect to each of measurement areas 1 to M obtained by dividingthe determination area i into M areas with a bandwidth m (M=n/m)sufficiently smaller than a grid width of a monitoring-target opticalsignal composing the wavelength-multiplexed optical signal, andoutputting M×N measured values; and determining that dummy lightcorresponding to the determination area i needs introducing if each of Mmeasured values in the determination area i is smaller than a thresholdvalue i, and, determining that dummy light corresponding to thedetermination area i does not need introducing if at least one of the Mmeasured values in the determination area i is equal to or larger thanthe threshold value i.

Advantageous Effects of Invention

According to the above-mentioned aspects of the present invention, it ispossible to make a judgment of necessity to introduce dummy light havinga highly accurate grasp of a change in an input level influencing a gaineven though a wavelength-multiplexed optical signal is composed ofoptical signals in which a plurality of frequency grids are mixed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a block configuration diagram of an optical signal monitor 10according to a first example embodiment.

FIG. 1B is a diagram to illustrate the operation of the optical signalmonitor 10 according to the first example embodiment.

FIG. 2 is a block configuration diagram of a wavelength multiplexingtransmitter 100 according to a second example embodiment.

FIG. 3 is an example of optical signals output from transmitters 210 to260 according to the second example embodiment.

FIG. 4 is an example of measurement steps of an OCM 500 according to thesecond example embodiment.

FIG. 5 is an example of threshold values 1 to 11 and areas 1 to 11 setby a CPU 600 according to the second example embodiment.

FIG. 6 is a circuit configuration diagram of a dummy light source 700according to the second example embodiment.

FIG. 7 is an example of a power spectrum of dummy light output from thedummy light source 700 according to the second example embodiment.

FIG. 8 is an operational flow of the wavelength multiplexing transmitter100 according to the second example embodiment.

FIG. 9 is an example of a power spectrum of a transmission signal outputfrom an optical coupler 800 according to the second example embodiment.

FIG. 10 is an example of a power spectrum of dummy light output from thedummy light source 700 according to the second example embodiment.

FIG. 11A is a diagram illustrating an output level without introducingcorrection light if optical signals of wavelengths λ3 to λ7 are lost inPatent Literature 1.

FIG. 11B is a diagram illustrating an output level with introducingcorrection light if optical signals of wavelengths λ3 to λ7 are lost inPatent Literature 1.

EXAMPLE EMBODIMENT

Example embodiments of the present invention will be described in detailbelow with reference to the drawings. The directions of the arrows inthe drawings represent one example, and do not limit directions ofsignals between blocks.

First Example Embodiment

A first example embodiment of the present invention will be described.FIG. 1A is a block configuration diagram of an optical signal monitoraccording to the present example embodiment. In FIG. 1A, an opticalsignal monitor 10 includes storage means 20, measurement means 30, anddetermination means 40.

The storage means 20 holds a threshold value i (i=1 to N) set for eachof N determination areas i (i=1 to N) having a bandwidth n set inaccordance with an average grid of dummy light. As illustrated in FIG.1B, the determination areas 1 to N are obtained by dividing a usage bandBL of optical signals that the optical signal monitor 10 monitors into Nareas with respect to each bandwidth n (50 GHz, for example)corresponding to an average grid (50 GHz, for example) of dummy lightintroduced when an optical signal is lost.

The measurement means 30 divides the bandwidth n in the determinationarea i into M areas with respect to each bandwidth m that issufficiently smaller than a grid width of a monitoring-target opticalsignal (here, m=n/M), and sets M measurement areas 1 to M. That is tosay, as illustrated in FIG. 1B, the measurement areas 1 to M areobtained by dividing the bandwidth n (50 GHz, for example) in thedetermination area i into M areas by the bandwidth m (5 GHz, forexample) that is equal to or smaller than one third of the grid width ofthe monitoring-target optical signal. The measurement means 30sequentially measures an optical intensity of an inputtedwavelength-multiplexed optical signal (a signal in which themonitoring-target optical signals are wavelength-multiplexed) in themeasurement areas 1 to M of all the determination areas 1 to N, andoutputs M×N measured values.

If each of M measured values in the determination area i is smaller thana threshold value i, the determination means 40 determines that dummylight corresponding to the determination area i needs introducing. Incontrast, if at least one of the M measured values in the determinationarea i is equal to or larger than the threshold value i, thedetermination means 40 determines that dummy light corresponding to thedetermination area i does not need introducing.

As described above, the optical signal monitor 10 according to thepresent example embodiment sets the determination areas 1 to Ncorresponding to the average grid of the dummy light, and determines anoptical intensity of a wavelength-multiplexed optical signal withrespect to each of the measurement areas 1 to M into which thedetermination area is finely divided in accordance with the grid widthof the monitoring-target optical signals. In this case, it is possibleto make a judgment of necessity to introduce dummy light properly havinga highly accurate grasp of a change in an input level influencing a gaineven though the wavelength-multiplexed optical signal is composed ofoptical signals in which a plurality of frequency grids are mixed.

Second Example Embodiment

A second example embodiment will be described. FIG. 2 is a blockconfiguration diagram of a wavelength multiplexing transmitter accordingto the present example embodiment. In FIG. 2 , a wavelength multiplexingtransmitter 100 includes six transmitters 210 to 260, an opticalwavelength multiplexer 300, an optical splitter 400, an optical channelmonitor (OCM) 500, a CPU 600, a dummy light source 700, and an opticalcoupler 800.

Each of the transmitters 210 to 260 generates an optical signal having apredetermined bandwidth with a center frequency within an optical signalusage band ranging from 193.525 THz to 192.975 THz that differs fromeach other. For example, the transmitters 210 to 260 generate opticalsignals having a bandwidth of 50 GHz with their center frequencies equalto 193.500, 193.450, 193.400, 193.3625, 193.300, and 193.275 THz,respectively. FIG. 3 illustrates one example of optical signals outputfrom the transmitters 210 to 260. As illustrated in FIG. 3 , thetransmitters 210 and 220 generate optical signals having a grid of 50GHz, the transmitters 230 and 240 generate optical signals having a gridof 37.5 GHz, and the transmitters 250 and 260 generate optical signalshaving a grid of 25 GHz.

The optical wavelength multiplexer 300 wavelength-multiplexes theoptical signals respectively output from the transmitters 210 to 260 andoutputs a WDM signal. As mentioned above, different frequency grids aremixed in the WDM signal output from the optical wavelength multiplexer300. It is possible to use, as the optical wavelength multiplexer 300,an arrayed waveguide grating (AWG) module, an optical coupler, aninterleaver, a wavelength selective switch (WSS), or a deviceconfiguration obtained by combining those devices.

The optical splitter 400 splits the WDM signal output from the opticalwavelength multiplexer 300, outputs one of the WDM signal to the opticalcoupler 800, and outputs the other of the WDM signal to the OCM 500. Theoptical splitter 400 according to the present example embodiment splitsthe WDM signal in the splitting ratio of approximately nine to one, andoutputs the former to the optical coupler 800, the latter to the OCM500.

The OCM 500 continuously, periodically measures the optical power of theWDM signal inputted from the optical splitter 400 at a frequency spacingthat is sufficiently smaller than the frequency grid of the opticalsignal outputs from the transmitters 210 to 260. The OCM 500 accordingto the present example embodiment performs sampling measurement on theoptical power of the WDM signal at 5 GHz steps for the frequency gridsof the transmitters 210 to 260 (50 GHz grid, 37.5 GHz grid, and 25 GHzgrid). FIG. 4 illustrates the measurement steps of the OCM 500 accordingto the present example embodiment.

In FIG. 4 , the OCM 500 measures the optical power of the inputted WDMsignal in 5 GHz basis for the measurement band ranging from 193.525 THzto 192.975 THz. Specifically, the OCM 500 first measures the opticalpower of the WDM signal ranging from 193.525 THz to 193.520 THz,subsequently measures the optical power of the WDM signal ranging from193.520 THz to 193.515 THz, and sequentially measures the optical powerof the WDM signal with respect to each 5 GHz up to 192.975 THz. The OCM500 outputs, to the CPU 600, measurement results of the optical power ofthe WDM signal for each 5 GHz (referred to as an optical power measuredvalue below).

If it is intended to detect the presence or absence of the opticalsignal more finely, it is also possible to measure it by a step smallerthan 5 GHz step. In contrast, if it is intended to decrease the numberof times of the measurement, it is possible to use 10 GHz step or thelike, for example, and set a larger step so that three or more opticalpower measured values can be obtained within each area descried below.

The CPU 600 determines whether or not the optical signals generated inthe transmitters 210 to 260 are included in the WDM signal, andintroduces dummy light if the optical signals are not included. Here,the CPU 600 divides the measurement band of the optical signal rangingfrom 193.525 THz to 192.975 THz with respect to each average grid of thedummy light to be introduced into the WDM signal, by which the CPU setsa plurality of areas having the same bandwidth. In the present exampleembodiment, since the dummy light source 700 described below outputs thedummy light with 50 GHz grid, the CPU 600 divides the measurement bandranging from 193.525 THz to 192.975 THz with respect to each 50 GHz fromthe larger frequency side, and sets eleven areas 1 to 11. The set areas1 to 11 are illustrated in FIG. 5 . The CPU 600 holds threshold values 1to 11 for the areas 1 to 11, and makes a judgment of necessity tointroduce a dummy signal by comparing the optical power measured valueinputted from the OCM 500 with the corresponding threshold value. FIG. 5illustrates an example of the threshold values with a dotted line. Inthe present example embodiment, the threshold value is set at the samevalue in each of the areas 4 to 6, and in each of the areas 7 to 11.

The CPU 600 according to the present example embodiment compares tenoptical power measured values by 5 GHz step within the area 1 (from193.525 THz to 193.475 THz) with the threshold value 1 set for the area1, respectively. If at least one of the ten optical power measuredvalues within the area 1 is equal to or larger than the threshold value1, the CPU 600 determines that the dummy light does not need introducinginto the area 1. In this case, for example, the CPU 600 turns off aswitch 741 in the dummy light source 700 described below, and blocks theoutput of the dummy light corresponding to the area 1. In contrast, ifeach of the ten optical power measured values within the area 1 issmaller than the threshold value 1, the CPU 600 determines that thedummy light needs introducing into the area 1. In this case, the CPU 600turns on the switch 741 in the dummy light source 700, and causes thedummy light corresponding to the area 1 to be introduced into the WDMsignal.

The dummy light source 700 generates and outputs the dummy lightcorresponding to each of the areas 1 to 11. FIG. 6 illustrates anexample of a circuit configuration diagram of the dummy light source700. The dummy light source 700 in FIG. 6 is configured by an opticalamplifier 710, an optical demultiplexer 720, eleven pieces of outputwaveguides 731 to 7311, and switches 741 to 7411. The amplifiedspontaneous emission (ASE) light amplified at the optical amplifier 710is demultiplexed in the optical demultiplexer 720 into the dummy lightwith the bands corresponding to the areas 1 to 11, and is output to theoutput waveguides 731 to 7311 respectively. The switches 741 to 7411 aredisposed respectively in the output waveguides 731 to 7311, andcontrolled by the CPU 600 so that the dummy light inputted into theoutput waveguide in which the switch is turned on may be output to theoptical coupler 800. An AWG module or the like can be used as theoptical demultiplexer 720.

FIG. 7 illustrates a power spectrum of the dummy light inputted into theoptical coupler 800 when all the switches 741 to 7411 are turned on bythe CPU 600. As illustrated in FIG. 7 , in the present exampleembodiment, the dummy light beams are inputted into the optical coupler800, each of which has a rectangular waveform that has a bandwidth of 50GHz with the center frequency positioned at the 50 GHz grid (193.500THz, 193.450 THz, . . . and 193.000 THz).

The optical coupler 800 couples the WDM signal inputted from the opticalsplitter 400 with the dummy light inputted from the dummy light source700, and outputs the coupled signal as a transmission signal. When apart of the optical signals output from the transmitters 210 to 260 islost, the dummy light is coupled that corresponds to the area where theoptical signal has been lost; consequently, the total power of thetransmission signal output from the wavelength multiplexing transmitter100 is kept approximately constant. It is preferable for a couplingratio of the optical coupler 800 to be set at approximately one to onebetween the WDM signal side and the dummy light side.

Next, the operation of the wavelength multiplexing transmitter 100 whenoptical signals from the transmitters 240 and 250 are lost will bedescribed using FIG. 8 and FIG. 9 . FIG. 8 illustrates an operationalflow of the wavelength multiplexing transmitter 100, and FIG. 9illustrates a power spectrum of transmission signals output from theoptical coupler 800.

In FIG. 8 , the optical wavelength multiplexer 300 in the wavelengthmultiplexing transmitter 100 wavelength-multiplexes optical signalsinputted from the transmitters 210 to 260, and outputs a WDM signal(S101). If the optical signals from the transmitters 240 and 250 arelost, only optical signals output from the transmitters 210 to 230, and260 are wavelength-multiplexed into the WDM signal. FIG. 9 illustrates,by stippled areas, the power spectrum of the WDM signal output from theoptical wavelength multiplexer 300.

The WDM signal output from the optical wavelength multiplexer 300 issplit in the ratio of approximately nine to one 9:1 at the opticalsplitter 400, and then the former is output to the optical coupler 800,and the latter is output to the OCM 500 (S102).

The OCM 500 continuously, periodically measures, at 5 GHz steps, theoptical power of the WDM signal inputted from the optical splitter 400,and outputs an optical power measured value to the CPU 600 (S103).

The CPU 600 compares the optical power measured values inputted from theOCM 500 with the corresponding threshold values 1 to 11 with respect toeach of the areas 1 to 11 (S104). If all of the ten optical powermeasured values in the area i are smaller than the correspondingthreshold value i (S104/YES), the CPU 600 determines that dummy lightneeds introducing into the area i, and turns on the switch 74 i in thedummy light source 700 corresponding to the area i (S105). In contrast,if at least one of the optical power measured values is equal to orlarger than the corresponding threshold value i (S104/NO), the CPU 600determines that dummy light does not need introducing into the area i,and turns off the switch 74 i in the dummy light source 700corresponding to the area i (S106).

Specifically, if the optical signals from the transmitters 240 and 250are lost, the CPU 600 determines that dummy light does not needintroducing into the area 1 because some of the ten optical powermeasured values in the area 1 are larger than the threshold value 1. Inthis case, the CPU 600 turns off the switch 741 corresponding to thearea 1 in the dummy light source 700. Similarly, the CPU 600 turns offthe switches 742 and 743 corresponding to the areas 2 and 3.

In contrast, because all of the ten optical power measured values in thearea 4 are smaller than the threshold value 4, the CPU 600 determinesthat dummy light needs introducing into the area 4. The CPU 600 thenturns on the switch 744 corresponding to the area 4 in the dummy lightsource 700. This causes the dummy light in the band corresponding to thearea 4 to be output from the dummy light source 700 to the opticalcoupler 800. In FIG. 9 , power spectra of the dummy light output fromthe dummy light source 700 to the optical coupler 800 are illustrated bythe shaded areas.

In addition, the CPU 600 compares the ten optical power measured valuesin the area 5 with the threshold value 5. Because a part of the opticalsignal from the transmitter 260, besides the optical signal from thetransmitter 250 that has been lost, is also included in the area 5, someof the ten optical power measured values in the area 5 become largerthan the threshold value 5. In this case, the CPU 600 determines thatthe dummy light does not need introducing into the area 5, and turns offthe switch 745 that corresponds to the area 5. After this, the CPU 600performs similar operations (introducing or blocking dummy light) forthe areas 6 to 11. That is to say, the CPU 600 turns off the switch 746that corresponds to the area 6, and turns on the switches 747 to 7411that correspond to the areas 7 to 11.

The optical coupler 800 couples the WDM signal inputted from the opticalsplitter 400 with the dummy light inputted from the dummy light source700, and outputs the transmission signal illustrated in FIG. 9 (S107).

As described above, the wavelength multiplexing transmitter 100according to the present example embodiment sets a plurality of areasthat correspond to average grids of the dummy light, and measures theoptical power of the WDM signal output from the optical wavelengthmultiplexer 300 in a bandwidth into which the area is finely divided. Ifall of measured values in the area are smaller than the threshold value,the dummy light with the band corresponding to the area is introducedinto the transmission signal, and the dummy light with the bandcorresponding to the area is blocked if at least one of measured valuesin the area is equal to or larger than the threshold value. This makesit possible to make a judgment of necessity to introduce dummy light andto correct the total power of the transmission signal output from thewavelength multiplexing transmitter 100 with a high degree of accuracy,even though optical signals in various frequency grids are included inthe WDM signal output from the optical wavelength multiplexer 300.

In the present example embodiment, the dummy light source 700 generatesthe dummy light having a rectangular waveform that has a bandwidth of 50GHz with the center frequency positioned at 50 GHz grid (FIG. 7 ), towhich the present invention is not limited. It is only necessary to havepredetermined optical power within the area, and the dummy light asillustrated in FIG. 10 can be used, for example. In FIG. 10 , eachoptical power (square measure) of the dummy light having a rectangularwaveform is set so as to become equivalent to the optical power (squaremeasure) of the optical signal output from the transmitter.

If there is a limit to introducing the dummy light, it is notnecessarily required to equalize the optical power of the dummy light tothe optical power of the optical signal. If the dummy light is forbiddento be introduced because bands on both sides of the optical signal areused as guard bands, for example, the power of the dummy light isslightly increased compared to the power of the optical signal, whichenables the total power of the transmission signal to be kept constant.

In the present example embodiment, each band of the areas 1 to 11 ismade to correspond to an average grid of the dummy light and constant(50 GHz); however, when the dummy light illustrated in FIG. 10 is used,it is also possible to set a bandwidth of each area arbitrarily and setthe center frequency of the area at a value different from the centerfrequency of the dummy light if it is possible to make the connectionbetween the dummy light and the area. It is not limited for the dummylight to correspond to the area by one-to-one; for example, two areascan be made to correspond to one of dummy light beams.

In addition, because the optical signal from the transmitter 250 andoptical signal from the transmitter 260 are included in the area 5 inthe present example embodiment, introduction of the dummy light into thearea 5 is blocked even though the optical signal from the transmitter250 is lost. However, for example, if the number of optical powermeasured values that are equal to or larger than the threshold value 5is equal to or less than two, a threshold value 5′ larger than thethreshold value 5 is set, and the two optical power measured values arefurther compared with the threshold value 5; as a result, the dummylight is introduced into the area 5 if all of the two optical powermeasured values are smaller than the threshold value 5′.

The present invention has been described above with reference to theabove-mentioned example embodiments as typical examples. However, thepresent invention is not limited to these embodiments. In other words,various forms understandable for those skilled in the art can be appliedto the present invention without departing from the scope of the presentinvention.

REFERENCE SIGNS LIST

-   -   10 Optical signal monitor    -   20 Storage means    -   30 Measurement means    -   40 Determination means    -   100 Wavelength multiplexing transmitter    -   210 to 260 Transmitter    -   300 Optical wavelength multiplexer    -   400 Optical splitter    -   500 OCM    -   600 CPU    -   700 Dummy light source    -   710 Optical amplifier    -   720 Optical demultiplexer    -   731 to 7311 Output waveguide    -   741 to 7411 Switch    -   800 Optical coupler

What is claimed is:
 1. An optical signal control apparatus, comprising: a controller configured to control a dummy light using an optical intensity of a wavelength-multiplexed optical signal at a wavelength spacing and a threshold value in a wavelength band corresponding to the optical intensity; and a storage configured to store the threshold value assigned to the wavelength band, wherein the wavelength spacing is less than the wavelength band.
 2. The optical signal control apparatus according to claim 1, wherein the controller is configured to determine the optical intensity of the wavelength-multiplexed optical signal with respect to each wavelength spacing into which the wavelength band is finely divided in accordance with a grid width of an optical signal to control the dummy light.
 3. The optical signal control apparatus according to claim 1, wherein the controller is configured to control to introduce the dummy light into the wavelength band according to the optical intensity.
 4. The optical signal control apparatus according to claim 3, wherein the controller is configured to determine, if each optical intensity in a predetermined wavelength band is smaller than the threshold value assigned to the predetermined wavelength band, that the dummy light needs introducing into the predetermined wavelength band.
 5. The optical signal control apparatus according to claim 3, wherein the controller is configured to determine, if at least one optical intensity in a predetermined wavelength band is equal to or larger than the threshold value assigned to the predetermined wavelength band, that the dummy light does not need introducing into the predetermined wavelength band.
 6. The optical signal control apparatus according to claim 1, wherein the wavelength-multiplexed optical signal comprises optical signals in which a plurality of frequency grids are mixed.
 7. An optical signal controlling method, comprising: controlling a dummy light using an optical intensity of a wavelength-multiplexed optical signal at a wavelength spacing and a threshold value in a wavelength band corresponding to the optical intensity; and storing the threshold value assigned to the wavelength band, wherein the wavelength spacing is less than the wavelength band.
 8. The optical signal controlling method according to claim 7, wherein the controlling of the dummy light includes determining the optical intensity of the wavelength-multiplexed optical signal with respect to each wavelength spacing into which the wavelength band is finely divided in accordance with a grid width of an optical signal to control the dummy light.
 9. The optical signal controlling method according to claim 7, wherein the controlling of the dummy light includes controlling to introduce the dummy light into the wavelength band according to the optical intensity.
 10. The optical signal controlling method according to claim 9, wherein the controlling of the dummy light includes determining, if each optical intensity in a predetermined wavelength band is smaller than the threshold value assigned to the predetermined wavelength band, that the dummy light needs introducing into the predetermined wavelength band.
 11. The optical signal controlling method according to claim 9, wherein the controlling of the dummy light includes determining, if at least one optical intensity in a predetermined wavelength band is equal to or larger than the threshold value assigned to the predetermined wavelength band, that the dummy light does not need introducing into the predetermined wavelength band.
 12. The optical signal controlling method according to claim 7, wherein the wavelength-multiplexed optical signal comprises optical signals in which a plurality of frequency grids are mixed. 